An output power level of a system is frequently a critical factor in the design and performance of almost all radio frequency and microwave equipment incorporating the system. For the measurement of certain power levels, power meters employing thermocouple sensors are considered preferable to power meters employing thermistor sensors since, in addition to possessing an inherent square-law detection characteristic, the thermocouple sensors exhibit a higher sensitivity and has a higher dynamic range of powers measured than the thermistor sensors.
However, one criticism of power measurements taken using thermocouple power meters is that the power meters that use thermocouples do not exercise direct power substitution. Consequently, a given measurement circuit can produce different DC output voltages as a result of sensitivity differences between different sensors being used with the given measurement circuit. Alternatively, or additionally, the different output voltages can be attributed to drift in the sensitivity of a given sensor unit due to aging of the thermocouple sensor of the given sensor unit or temperature effects. Since power meter systems using thermocouple sensors do not possess a feedback path to enable correction for differences in sensor sensitivities, aging of the thermocouple sensors, or temperature effects, power measurements taken with thermocouple sensors are said to be uncorrected and are, hence, considered less accurate than power measurements taken using thermistor power meters. In contrast, power meters using thermistor sensors employ a so-called DC-substitution process.
In order to overcome the above shortcomings of power meters that use thermocouple sensors, it is known to equip the power meters with a power reference oscillator, typically operating at a frequency of 50 MHz and an output power of 1 mW. If a user of the power meter needs to verify the accuracy of the power meter, thermocouple sensor and cable coupled in-between (hereinafter referred to as the “power measurement system”), or adjust the power meter for a sensor of a different sensitivity, the user can connect the thermocouple sensor to an output of the power reference oscillator of the power meter and, using a calibration adjustment, set the power meter to read a known value of a reference power generated by the power reference oscillator, for example 1.00 mW, or a value derived from the known value of the reference power. The overall accuracy of the power measurement system is dependent upon the accuracy of the power reference oscillator.
The reference oscillator is therefore clearly an important part of power measurement systems, especially those using thermocouple or diode sensors, if accuracy of power measurements is to be maintained. One important parameter of the reference oscillator which needs to be maintained is an output impedance of the reference oscillator in order to ensure that the impedances of the meter/cable/sensor combination and the reference oscillator are matched so that a correct amount of RF power is transferred from the reference oscillator to the power measurement system.
In the context of testing the output impedance of the reference oscillator, it therefore follows that the transfer of RF power from a source (the reference oscillator) to a test load (a test power meter/cable/sensor combination) is dependent upon the impedance of the load and the effective impedance of the source. The RF power, PA, transferred can be expressed in terms of Voltage Reflection Coefficients (VRCs):                               P          A                =                              P                          Z              0                                ×                                    1              -                                                                                      Γ                    2                                                                    2                                                                                                      1                  -                                                            Γ                      S                                        ⁢                                          Γ                      L                                                                                                  2                                                          (        1        )            where:                PZ0 is the power that the source can deliver to a load equal to Z0Ω; and        Γs and ΓL are the respective, complex number, VRCs of the source and the load.        
However, in most cases, only magnitudes of the source VRC and the load VRC are known, and so only upper and lower limits can be placed on the transferred power level, PA. Consequently, an uncertainty as to the exact amount of power transferred from the reference oscillator to the test power meter exists. Mathematically, the uncertainty, U, can be expressed approximately as follows:U=±200|ΓSΓL|%  (2)
As an example, for a known reference oscillator of an Agilent Technologies, Inc. E4418B power meter generating an output signal having a frequency of 50 MHz at a power level of 1 mW, an overall uncertainty attributable to a measurement of the power level of the output signal is of the order of ±0.5% to 0.7%. Approximately, 0.1% to 0.3% of the overall uncertainty can be attributed to the mismatch uncertainty associated with the transfer of the power of the reference oscillator output signal to the test power meter. The mismatch uncertainty can therefore make a significant contribution to the overall uncertainty attributable to the measurement of the power level of the output signal mentioned above. Given the purpose served by reference oscillators, and more generally power reference sources, the mismatch uncertainty therefore needs to be maintained at a very low value. There is therefore clearly a need to calculate the mismatch uncertainty associated with the amount of power transferred from the reference oscillator to the test power meter, and so values for the load and source VRCs, ΓL, ΓS, need to be measured or calculated.
Whilst the VRC of the load (the test meter system) can be measured with relative ease using conventional methods, the VRC of the source (the reference oscillator) can only be established when the reference oscillator is active. As a result of the need for the reference source to be active, known measurement techniques, such as Vector Network Analysis (VNA), can not be used, because VNA relies upon a principle of applying a sinusoidal signal to the reference oscillator and measuring relative amplitude and phase of a reflected signal; the reflected signal will be distorted by the signal generated by the reference oscillator. It will therefore be impossible to measure accurately the proportion of the VNA signal that has been reflected, as the VNA signal and the signal generated by the reference oscillator are at a same frequency.
Another known technique is a so-called impedance bridge for measuring the output impedance of the reference oscillator. However, the signal generated by the reference oscillator will also interact with the impedance bridge in such a way as to render any measurement of the impedance of the reference oscillator inaccurate.
In order to obviate the above disadvantage of the impedance bridge measurement technique, another known measurement technique disclosed in the Agilent E4416A/E4417A Service Guide (Agilent Technologies Limited, E4416-90014) provides a way of measuring the source match using a modified impedance bridge. In particular, the technique employs an Agilent Technologies Inc 432A power meter having a self-adjusting impedance bridge, one branch of the impedance bridge comprising thermistors, the impedance of which can be altered by altering a switchable resistance, in an adjacent branch, between two settings: 200Ω and 100Ω. The switching of the resistance results in a consequential change in the impedance of the thermistors, and hence the VRC of the test meter system, or load. Consequently, a mechanism is provided to switch the load VRC, ΓL, between two values, allowing the source VRC, ΓS, to be calculated approximately, and verified by measurement using conventional means. Once the source VRC, ΓS, is known, it is possible, using equation (2), to calculate the mismatch uncertainty, U, associated with the transfer of power from the reference oscillator to the 432A test power meter and with the adjustment of the output power of the reference oscillator.
However, some thermistors employed in sensor units have a negative temperature coefficient with respect to resistance. As a result, the temperature of the thermistor(s) rise(s) when the resistance of the thermistor(s) fall(s). This temperature rise can make the sensor unit less reliable, or can even damage the thermistor(s) and hence the sensor unit.